Figures

Abstract

Background

Idiopathic congenital talipes equinovarus (CTEV) is a common developmental
foot disorder, the aetiology of which remains largely unknown. Some aspects
of the epidemiology suggest the possibility of aetiologically distinct
subgroups. Previous studies consider CTEV as a homogenous entity which may
conceal risk factors in particular subgroups. We investigate evidence for
aetiologically distinct subgroups of CTEV.

Methods

Parents of 785 probands completed a postal questionnaire. Family pedigrees
were compiled by telephone. Case-only analysis was used to investigate
interactions between risk factors and sex of the proband, CTEV laterality
and CTEV family history.

Results

The male∶female ratio was 2.3∶1, 58% of probands were
affected bilaterally and 11% had a first-second degree family
history. There were modest interactions between family history and twin
births (multivariate case - only odds ratio
[ORca] = 3.87, 95%CI 1.19–12.62)
and family history and maternal use of folic acid supplements in early
pregnancy (ORca = 0.62, 95%CI 0.38–1.01);
and between sex of the proband and maternal alcohol consumption during
pregnancy (female, positive history and alcohol consumed:
ORca = 0.33, 95%CI 0.12–0.89). Previous
reports of an interaction between maternal smoking and family history were
not confirmed. Relatives of female probands were affected more often than
relatives of male probands.

Conclusions

These results provide tentative evidence for aetiologically distinct CTEV
subgroups. They support the ‘Carter effect’, suggesting CTEV
develops though a multifactorial threshold model with females requiring a
higher risk factor ‘load’, and suggest areas where future
aetiological investigation might focus. Large multi-centre studies are
needed to further advance understanding of this common condition.

Funding: This work was supported by Sports Action Research for Kids (Sparks) and the Chief
Scientist Office. The funders had no role in study design, data collection and
analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Congenital talipes equinovarus (CTEV) is a common developmental disorder with birth
prevalence of 1–4.5 per 1000.[1] Affected feet are inclined inwards, axially rotated
outwards, and point downwards, with concomitant soft tissue abnormalities.[2] Severity ranges
from cases that resolve with manipulation to those requiring multiple operations
with disability and discomfort persisting into later life. Although some cases occur
with other neuromuscular and neurological disorders, most affected children have
idiopathic CTEV.[3]

Mechanical, neurological, muscular, bony, connective tissue and vascular mechanisms
for idiopathic CTEV have all been proposed.[3] Although genetic and
lifestyle/environmental factors are thought to be aetiologically relevant, the
genetic model is unclear and little is known about non-genetic risk factors.[3] However,
some aspects of the epidemiology suggest areas worthy of further study: twice as
many males as females are affected[4]–[7] and there is evidence of the
‘Carter effect’ (higher risk in relatives of affected females);[8], [9]
7–21%[10], [11] of families report CTEV in first-degree relatives, and
one study suggests that family history modifies the association between CTEV and
maternal smoking;[10] around half of affected children have bilateral CTEV[1], [4], [12], [13] and mouse
studies suggest the number of affected feet is a marker for genetic load.[14] These
observations raise the possibility of aetiologically distinct CTEV subgroups.
Previous studies have considered idiopathic CTEV as a homogenous entity that may
have concealed risk factors relevant, or more important, in particular
subgroups.

The ECCE (Exploring Causes of Clubfoot in Europe) study comprises the largest
reported series of idiopathic CTEV with primary data collection. Here, we
investigate interactions between epidemiological risk factors and family history,
the proband's sex, and laterality of the condition. We also report family
pedigree analyses.

Methods

Ethics Statement

The Grampian Research Ethics Committee approved the study and written consent was
obtained from each participating family (most often the mother signed on behalf
of her partner and participating children).

Subjects

Subjects were recruited May 2001–May 2003 through two support groups,
steps[15]
in the United Kingdom and VOK[16] in the Netherlands. The
support groups approached families by mail on behalf of the investigators. A
parent of the affected child (generally the mother) completed a questionnaire
that included: nature of the condition (laterality, treatment, other medical
conditions), maternal reproductive history, parental lifestyle (tobacco,
alcohol, folic acid supplement and oral contraceptive [OC] use in the
periconceptional period of the index pregnancy), and CTEV family history. On
questionnaire receipt, a clinical geneticist (ZM) reviewed details of the foot
defect and any additional conditions to exclude syndromic cases and non-CTEV
conditions. Pedigrees were elicited by telephone from families who reported CTEV
in family members other than the proband.

Statistical analysis

The analysis included unrelated index children with idiopathic CTEV. Case-only
methods[17], [18] were used to investigate whether CTEV risk factors
differed by presence/absence of CTEV family history; sex of the proband; or
laterality of the condition. Analysis contrasted sub-groups of cases with
particular combinations of these “stratification variables” and risk
factor exposures (e.g. male/female proband and maternal folic acid use/non-use),
with the “association” between the stratification variable and risk
factor (strictly the interaction, or departure from a multiplicative
relationship) expressed as a case-only odds ratio (ORca). The stratification
variables reference categories were: no family history; male; and unilateral
CTEV. The primary analysis concerned first or second-degree family history.
Using logistic regression, a “minimally adjusted” ORca was computed
for each risk factor adjusted for country. Factors where the likelihood ratio
test (LRT) p value was ≤0.1 in minimally adjusted analysis were considered
for inclusion in multivariate models. Final multivariate models included country
and variables where p≤0.1 for the LRT comparing the multivariate model
containing the variable with the model that did not. The family history analysis
was repeated stratifying by sex, since sex differences have been reported.[10]

Using the pedigrees, the total numbers of affected and unaffected first and
second- degree relatives were determined. The ratio of affected to total
relatives was calculated overall and by sex of the relative, proband, and
relative and proband. Associations were assessed using the chi-square test.

Results

Participant characteristics

The male∶female ratio was 2.3∶1 (Tables 1, 2). More than half had bilateral CTEV
(58%). In unilateral cases the right foot was affected most often
(56% right, 44% left). CTEV in first-second degree family members
was reported by 11% of families, in first-third degree relatives by
16% and in ‘any’ family member by 26%.

Family history associations

Factors that interacted with first-second degree family history in relation to
CTEV risk were: maternal OC use, maternal use of folic acid-containing
supplements, maternal ethnicity, twin birth and birthweight (Tables 3, 4, 5). Compared to those with no family history,
probands with a family history were more likely to have a twin, have mothers who
were non-Caucasian, and have mothers who took OCs in early pregnancy; they were
less likely to have mothers who took folic acid supplements
periconceptionally.

Maternal smoking in the periconceptional period was less common in those with a
family history, reflected in an inverse, but non-statistically significant, ORca
(multivariate ORca = 0.64, 95%CI 0.34–1.22,
p = 0.16). The risk estimates were similar for smoking in
the three months pre-conception and in the first trimester (data not shown).
There was no association with paternal smoking.

After stratifying by sex, males with a family history were more likely than those
without to have mothers who took OCs in early pregnancy (multivariate
ORca = 4.35, 95%CI 1.01–18.78,
p = 0.07) and to have a twin
(ORca = 5.28, 95%CI 1.31–21.32,
p = 0.03), and less likely to have
mothers who took folic acid-containing supplements first trimester
(ORca = 0.59, 95%CI 0.31–1.10,
p = 0.10) or who had previously had a
miscarriage (ORca = 0.53, 95%CI 0.24–1.17,
p = 0.10). Birthweight distribution
varied between males with and without a family history (<2500 g
ORca = 0.66 95%CI 0.15–2.89; 2500–2999 g
ORca = 1.12, 95%CI 0.42–2.99; 3000–3499
g ORca = 1.00 [reference]; 3500–3999 g
ORca = 0.43, 95%CI 0.18–1.03; ≥4000 g
ORca = 1.57 95%CI 0.70–3.55;
p = 0.07). Females with a family
history were less likely than those without to have been delivered by caesarean
section (ORca = 0.23, 95%CI 0.03–1.86,
p = 0.10) and to have mothers who
consumed alcohol (ORca = 0.33, 95%CI
0.12–0.89, p = 0.02) or had an
infection (ORca = 0.11, 95%CI 0.01–0.95,
p = 0.01) during pregnancy. They were
more likely to have mothers who were non-Caucasian
(ORca = 16.18, 95%CI 1.19–220.5,
p = 0.03) and who had an amniocentesis
in the index pregnancy (ORca = 5.69, 95%CI
1.46–22.15, p = 0.02).

Associations by proband sex

The factors which interacted with sex to affect CTEV risk were: maternal
gravidity and miscarriage history, chorionic villus sampling in the index
pregnancy, forceps delivery, birthweight, and proband birth year. Compared to
males, females were more likely to have mothers who were multiparous (two
pregnancies: multivariate ORca = 2.46, 95%CI
1.40–4.30; ≥three pregnancies: ORca = 1.98,
95%CI 1.08–3.66, p = 0.005),
had a history of miscarriage (ORca = 1.42, 95%CI
0.94–2.12, p = 0.09), and had
chorionic villus sampling in the index pregnancy
(ORca = 3.27, 95%CI 0.90–11.90,
p = 0.07). Females were less likely to
have been delivered by forceps (ORca = 0.31, 95%CI
0.13–0.77, p = 0.01), were lighter
at birth and were more likely to be born in earlier years (data not shown).

Associations by CTEV laterality

The factors which interacted with laterality to affect CTEV risk were: gestation,
maternal gravidity and alcohol consumption, and family history. Compared to
unilateral CTEV, probands affected bilaterally were less likely to have been
premature (multivariate ORca = 0.51, 95%CI
0.24–1.09, p = 0.07) and to have
mothers who consumed alcohol during pregnancy (ORca = 0.76,
95%CI 0.56–1.03, p = 0.07),
but more likely to have a first- third degree family history
(ORca = 1.43, 95%CI 0.95–2.14,
p = 0.08) and to have mothers who had
two pregnancies in total (one pregnancy ORca 1.00 [reference], two
pregnancies ORca = 1.38, 95%CI 0.89–2.12;
≥three pregnancies ORca = 0.90, 95%CI
0.59–1.39; p = 0.03).

Pedigree analysis

CTEV in first-degree relatives was reported in 5.7% (45/785) of families;
5.7% (45/785) had affected second-degree relatives, 1.0% (8/785)
had affected first and second-degree relatives and 10.5%
(82/785) had affected first or second-degree relatives. Of
those with a first-degree family history, 38 had one affected relative (15 sibs,
14 fathers, nine mothers), six had two affected relatives (three
sib/mother-pairs, one sib/father-pair and two sib-pairs) and one had three
affected relatives (mother and two sibs). Regardless of degree of relatedness,
139 families reported one affected relative, 46 reported two, 13 reported three,
two reported four and five reported five.

Discussion

Strengths and limitations

Most previous CTEV studies have either been based on routine data, which gives
large sample sizes but lack certainty about the diagnosis of CTEV, or on small
clinical series from single centres, which may be highly selected. In addition,
studies do not always distinguish clearly between syndromic and idiopathic CTEV.
The current study is the largest reported series of idiopathic CTEV involving
primary data collection, and we carefully reviewed questionnaires to exclude
syndromic CTEV and other foot conditions. The case-only design is statistically
powerful for the investigation of interactions.[17], [18] The key assumption
underpinning the design is independence in the population between the
stratification variable and risk factor;[19] if violated, risk estimates
may be biased. We are not aware of any evidence to suggest the factors
considered are not independent.

Recall accuracy and diagnostic reliability are challenges in family history
analyses. We confirmed positive reports by telephone interview and additional
questionnaires where possible, and restricted most analyses to first-second
degree history, which may be more accurately reported.

Study participants were accrued from two national support groups, raising the
possibility that they might not be representative of all idiopathic CTEV. For
the results to be seriously biased, the probability of participation would need
to have been associated with family history, laterality or proband sex. The sex
ratio and laterality distribution mirrors patterns seen elsewhere.[1], [4], [6], [11], [20]–[27] Moreover, the proportion with a family history
corresponds with the upper limit of estimates from two US series,[4], [28] is
consistent with the UK Talipes series,[26] and is slightly lower that
in series of 120 Scottish children.[22] This suggests our
results are unlikely to be seriously biased.

Parental smoking

Reports of associations between foot deformities, including CTEV, and maternal
smoking during pregnancy are inconsistent.[5], [7], [10], [29]–[33] One US
case-control study of idiopathic CTEV reported a greater than multiplicative
interaction between smoking and family history, such that maternal smoking
increased risk only in children with a family history (OR 20.30, 95%CI
7.90–52.17).[10] We, in contrast, found no evidence of any interaction
between family history and maternal (or paternal) smoking in the three months
before, or first trimester of, the index pregnancy. If anything our risk
estimates suggested a less than multiplicative interaction, although they were
not statistically significant.

In our study maternal smoking prevalence in the first trimester was 15%
(22% in the three months before the pregnancy or first trimester)
compared with 38% in the first trimester among cases in the US study.
This difference could be due to differences in data collection methods
(interview versus postal questionnaire), study location or subjects' period
of birth (1968–1980 vs 1941–2003 [>70%
1991–2000]). The US study defined family history as
‘probable’ CTEV in first-degree relatives, but when we restricted
our analysis to first-degree relatives and first trimester smoking the risk
estimate was further from unity (multivariate ORca = 0.59,
95%CI 0.21–1.69, p = 0.30). The CTEV-smoking
relationship, in those with or without a family history, thus remains
controversial, and a role for smoking in CTEV cannot be entirely ruled out.

Perinatal factors and other maternal exposures during index pregnancy

The observed significant (p = 0.04) interaction between a
positive family history and twin births is novel and may have become evident
because, unlike previous studies of CTEV and twinning,[4], [34] we stratified by family
history. It could be interpreted as consistent with the uterine constraint
hypothesis for CTEV.[3]

As with other congenital anomalies,[35] there is some evidence of a
role for folate metabolism in CTEV.[13], [36], [37] The borderline significant
interaction between family history and maternal folic acid supplement use
(p = 0.09) provides some further support for this. Although
recall accuracy might be a concern, it seems unlikely this would be differential
by family history. Since our results suggest supplement use might be associated
with reduced CTEV risk in those without a family history further investigation
is warranted.

Although observed in a subgroup analysis, the significant interaction
(p = 0.02) between maternal alcohol consumption and family
history in females is intriguing (mothers of female probands with a family
history were less likely to report alcohol consumption). It is unlikely the
finding reflects avoidance of ‘risky’ behaviour during pregnancy in
women aware of a family history, as the association was not seen in males.
Although alcohol is teratogenic,[38] it has rarely been considered in relation to CTEV and
further investigation would be valuable.

The suggestion of an interaction between family history and maternal OC use in
early pregnancy is of interest, especially as the effect was strongest in males.
Increased risk of congenital limb deficiencies in offspring of mothers who had
taken relatively high-dose OCs in the periconceptional period has been
reported,[39] suggesting our finding could be due to specific OC
types (e.g. higher-dose or anti-androgenic OCs). We could not explore further as
we did not have information on types of OCs used. However, while some studies
report modest increased risks of birth defects, including limb deformities, with
OC use,[40]
the FDA concluded they were not teratogenic[41] and it is unclear how much
of the maternal hormones reach the fetus and whether exogenous hormones are more
likely to cross the placental barrier than endogenous (P Fowler, personal
communication). Moreover, since our result was only borderline significant it
may be due to chance.

Carter effect

Our results add to growing evidence for the Carter effect and a multifactorial
threshold model in CTEV. The observed higher CTEV risk in relatives of female
probands is consistent both with early work from Wynne-Davis et al, based on 144
UK cases born in 1940–1961,[8] and a recent US study
which described increased CTEV transmission from mothers to their offspring
compared with fathers.[9] Although other studies found CTEV risk was independent
of the proband's sex, these included relatively few pedigrees
(n<175).[4], [26] The somewhat different risk factor pattern in females
and males also points towards the possibility that a higher “load”
of risk factors (whether genetic and/or environmental) in families of affected
girls might predispose to CTEV.

Conclusions

Using the largest series of idiopathic CTEV with primary data collection so far
reported, we set out to (1) follow-up previous observations suggesting the
possibility of risk factor heterogeneity and (2) generate hypotheses for future
study. Our results provide support for the ‘Carter effect’,
suggesting that females require a higher risk factor ‘load’ before
developing CTEV. Beyond this, although we found only tentative evidence for
aetiologically distinct subgroups, our results do suggest some areas worth
further exploration, including the relationships between family history and
twinning and maternal use of folic acid supplements and alcohol during the index
pregnancy. Large multi-centre studies, with sufficient power to fully explore
risk factors in different case sub-groups, are needed to further elucidate the
aetiology of this common, but poorly understood, condition.

Acknowledgments

Thanks to Simon Barker and Professor Paul A Fowler for invaluable advice, Anne-Marie
Fegen for help with translation, Anne-Marie Fegen and Hazel Hailey for obtaining the
pedigrees and phoning the mothers for clarification of diagnoses, Martine Barnes for
managing the study so ably, and to Sue Banton, Marjolijn Kaminski and staff and
members of the family groups STEPS and VOK, without whom the study could not have
taken place.

Author Contributions

Conceived and designed the experiments: LS ZM RCH. Performed the experiments:
AHC NT. Analyzed the data: AHC NT. Wrote the paper: AHC LS ZM NT RCH.